Pythagoras believed that "all is numbers" and they maintained that all numbers can be expressed as a fraction, then Hippasus (maybe) showed some numbers cannot be expressed that way. Pythagorus thought this idea was stupid and he had him killed, but this could not stop the eventual collapse of his school.
This question could be too subjective (I'll remove it if it is), but is this story untrue or too obscure to know owing to the secretive nature of his sect and its ancient nature?